This invention relates generally to a method for obtaining and storing information about physical parameters or phenomena detected in a well bore and more particularly, but not by way of limitation, to a method for controlling the rate at which a downhole logging tool samples and records a detected condition within a well bore in response to a predicted value of an n-th sample and the actual value of the n-th sample.
It is well known in the art that there is a need for apparatus which can measure environmental conditions or physical phenomena, such as pressure and temperature, in downhole locations within a well bore. The Bourdon tube is well known and has been used for many years to mechanically record pressure by means of a chart scribed on a metallic plate in response to pressure. Electronic recording gauges have also been used in well bores. In one type of electronic gauge, the information is obtained and substantially immediately transmitted to the surface, such as over a wire line, for real-time display. Another type of electronic gauge obtains and stores the information within itself downhole for use only after the gauge has been extracted from the well bore.
Although several different types of gauges have been proposed or used in the industry, we are not aware of any gauge which accommodates several different transducers having different output signal characteristics. Nor are we aware of a microprocessor-based gauge which monitors itself to insure it is operating within normal limits, and which permits sample rates and resolutions to be varied in response to both software and hardware monitored changes in the downhole pressure (or other monitored condition) and to changes in battery life and remaining memory capacity, and which can be automatically selectively powered down, and which has increased storage capacity achieved with highly reliable, duty-cycled for low power consumption, magnetic core memory densely packed in a folded configuration. Likewise, we do not know of any such apparatus which permits such increased storage capacity to be accessed with a bit-by-bit technique that reduces the chances of losing complete words of stored information should there be a failure within the storage elements. We are not aware of any such apparatus which can determine which, if any, memory cells are inoperable and which thereafter does not attempt to store information in those bad memory cells. Although to our knowledge there is not a downhole apparatus having each of the foregoing features, such an apparatus is needed because of the following advantages which are or can be obtained by appropriately implementing these features as is done in the present invention.
A gauge which accommodates different types of transducers is needed to provide flexibility of use since a single downhole gauge is used in different locations where certain types of transducers may not be available and for different customers who may specify different types of transducers.
Self-monitoring is an important feature in a microprocessor-based gauge because it enables the microprocessor to be reset should the microprocessor operate outside normal operating limits. This insures accurate data collection.
The software and hardware features by which sample rates and resolutions of the samples can be changed are important for at least two reasons. The software monitoring is important because it effectively increases the length of time over which samples can be obtained by reducing the sample rate when there is little or no change between or among consecutive samples of a parameter or when changes are substantially linear. The hardware monitoring is important because it detects, and forces the gauge to record, rapid changes which occur between the software set sample times and which thus would otherwise be lost. Adjusting the sample rate based upon the remaining battery life and memory insures that meaningful information is always obtained and properly stored. Theoretically, it is desirable to slow the sample rate sufficiently so that samples are obtained and stored without ever totally exhausting the battery life or the memory capacity prior to the time the apparatus is withdrawn from the well bore and deactivated. Having a selectable resolution is important so that suitable precision is obtained at each sample rate.
The feature of automatically selectively powering down selectable parts of the apparatus is important because it conserves the remaining battery life. Section selectability maximizes the conservation at any one time by powering only those sections which need to be operated at that time. Selective power down of substantially the entire tool both at scheduled times and at unscheduled times when nothing significant is happening further assists in energy conservation.
Having an increased storage capacity is critical in an apparatus which is to be placed downhole and left for extended periods of time without having the information immediately transmitted to the surface. Because monitoring which needs to be done in a downhole environment might extend over several hours or days, a large storage capacity is needed to retain all the necessary samples required to perform the analyses which are to be made with the information as known to the art.
The specific bit-by-bit technique for using the memory in the downhole environment is important to prevent lost information and, therefore, to prevent lost time and money in obtaining valid samples. Knowing those locations within a memory device which are inoperable at the time the memory is made or subsequently tested, but prior to introducing it into the downhole environment, is advantageous so that, once the device is downhole, information will not be written into, and thus not be lost from, such bad locations, preventing lost time and money in obtaining valid samples.
Although there are several types of gauges which have been proposed or used for recording information in a downhole environment, we are not aware of one which meets each of the foregoing needs in a single apparatus.